Multilayer coating for flame retardant foam or fabric

ABSTRACT

A method includes coating a substrate to provide a flame resistant substrate. In an embodiment, the method includes exposing the substrate to a cationic solution to produce a cationic layer deposited on the substrate. The cationic solution includes cationic materials. The cationic materials include polymers, nanoparticles, or any combinations thereof. The method further includes exposing the cationic layer to an anionic solution to produce an anionic layer deposited on the cationic layer to produce a bilayer. The bilayer is the anionic layer and the cationic layer. The anionic solution includes layerable materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application that claims thebenefit of U.S. Application Ser. No. 61/157,395 filed on Mar. 4, 2009,which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under grant number60NANB8D8104 by the National Institute of Standards and Technology. TheGovernment has certain rights in this invention.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to the field of coatings and more specifically tothe field of flame retardant coatings for substrates of foam or fabric.

Background of the Invention

Fire-related occurrences have caused widespread property damage andinjuries. It is well known that a wide range of commonly used materialsare flammable. To reduce the hazards from such flammable materials,flame retardants have been developed. Such flame retardants includehalogenated materials. Halogenated materials typically includebrominated compounds and phosphinated compounds. Drawbacks to suchhalogenated materials include the potential for haun to the environmentand humans. For instance, such halogenated materials may form toxins.Other drawbacks include a lack of durability that may be typical in someinstances to the brominated compounds.

The use of nanoparticles have been developed to overcome such drawbacks.However, drawbacks to use of nanoparticles include increased processingviscosity and modulus of the final polymer material, such as foam orfabric. Further drawbacks include inadequate flame suppression andmelt-dripping.

Consequently, there is a need for an improved fire retardant polymermaterial. There is a further need for improved fire retardant coatingsfor foam, fabric and other substrate materials.

BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS

In an embodiment, these and other needs in the art are addressed by amethod for coating a substrate to provide a flame resistant substrate.The method includes exposing the substrate to a cationic solution toproduce a cationic layer deposited on the substrate. The cationicsolution comprises cationic materials. The cationic materials include apolymer, nanoparticles, or any combinations thereof. The method alsoincludes exposing the cationic layer to an anionic solution to producean anionic layer deposited on the cationic layer. The depositionproduces a bilayer comprising the cationic layer and the anionic layer.The anionic solution comprises layerable materials.

In embodiments, these and other needs in the art are addressed by amethod for coating a substrate to provide a flame resistant substrate.The method includes exposing the substrate to an anionic solution toproduce an anionic layer deposited on the substrate. The anionicsolution includes layerable materials. The method also includes exposingthe anionic layer to a cationic solution to produce a cationic layerdeposited on the anionic layer. The deposition produces a bilayercomprising the anionic layer and the cationic layer. The cationicsolution includes cationic materials. The cationic materials include apolymer, nanoparticles, or any combinations thereof.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other embodiments for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent embodiments do not departfrom the spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawings inwhich:

FIG. 1 illustrates a coated substrate embodiment;

FIG. 2 illustrates an embodiment with bilayers of layerable materialsand additives;

FIG. 3 illustrates an embodiment with alternating layers of layerablematerials and additives;

FIG. 4 illustrates an embodiment with bilayers of layerable materialsand additives;

FIG. 5 illustrates film thickness as a function of the number ofdeposited bilayers;

FIG. 6 illustrates film mass as a function of individually depositedclay and polymer layers;

FIG. 7a ) illustrates weight loss as a function of temperature;

FIG. 7b ) illustrates weight loss as a function of temperature;

FIG. 8 illustrates X-ray diffraction patterns for fabric;

FIG. 9a ) illustrates weight loss as a function of temperature;

FIG. 9b ) illustrates weight loss as a function of temperature;

FIG. 10a ) illustrates absorbance as a function of deposited bilayers;

FIG. 10b ) illustrates absorbance as a function of deposited bilayers;

FIG. 11a ) illustrates weight loss as a function of temperature;

FIG. 11b ) illustrates weight loss as a function of temperature;

FIG. 12 illustrates SEM images of bare and coated foams;

FIG. 13 illustrates SEM images of foams after heat treatment; and

FIG. 14 illustrates weight loss as a function of temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an embodiment, a multilayer thin film coating method provides asubstrate with a fire retardant coating by alternately depositingpositive and negative charged layers on the substrate. Each pair ofpositive and negative layers comprises a bilayer. In embodiments, themultilayer thin film coating method produces a plurality of bilayers onsubstrates. The positive and negative layers may have any desiredthickness. In embodiments, each layer is between about 1 nanometer andabout 100 nanometers thick.

Any desirable substrate may be coated with the multilayer thin filmcoating method. In embodiments, the substrate includes foam or fabric.Any desirable foam may be used as the substrate. Without limitation,examples of suitable foams include polyurethane foam and polystyrenefoam. The fabric used may include any desirable type of fabric. Withoutlimitation, examples of suitable fabric include wool, linen, and cotton.

The negative charged (anionic) layers comprise layerable materials. Thelayerable materials include anionic polymers, colloidal particles, orany combinations thereof. Without limitation, examples of suitableanionic polymers include branched polystyrene sulfonate (PSS),polymethacrylic acid (PMAA), polyacrylic acid (PAA), or any combinationsthereof. In addition, without limitation, colloidal particles includeorganic and/or inorganic materials. Further, without limitation,examples of colloidal particles include clays, colloidal silica,inorganic hydroxides, silicon based polymers, polyoligomericsilsesquioxane, carbon nanotubes, graphene, or any combinations thereof.Any type of clay suitable for use in an anionic solution may be used.Without limitation, examples of suitable clays include sodiummontmorillonite, hectorite, saponite, Wyoming bentonite, halloysite, orany combinations thereof. In an embodiment, the clay is sodiummontmorillonite. Any inorganic hydroxide that may provide flameretardancy may be used. In an embodiment, the inorganic hydroxideincludes aluminum hydroxide, magnesium hydroxide, or any combinationsthereof.

The positive charge (cationic) layers comprise cationic materials. Thecationic materials comprise polymers, colloidal particles,nanoparticles, or any combinations thereof. The polymers includecationic polymers, polymers with hydrogen bonding, or any combinationsthereof. Without limitation, examples of suitable cationic polymersinclude branched polyethylenimine (BPEI), cationic polyacrylamide,cationic poly diallyldimethylammonium chloride (PDDA), or anycombinations thereof. Without limitation, examples of suitable polymerswith hydrogen bonding include polyethylene oxide, polyallylamine, or anycombinations thereof. In addition, without limitation, colloidalparticles include organic and/or inorganic materials. Further, withoutlimitation, examples of colloidal particles include clays, layereddouble hydroxides (LDH), inorganic hydroxides, silicon based polymers,polyoligomeric silsesquioxane, carbon nanotubes, graphene, or anycombinations thereof. Without limitation, examples of suitable layereddouble hydroxides include hydrotalcite, magnesium LDH, aluminum LDH, orany combinations thereof.

In embodiments, the positive and negative layers are deposited on thesubstrate by any suitable method. Without limitation, examples ofsuitable methods include bath, spray, or any combinations thereof. In anembodiment, the positive and negative layers are deposited by bath.

FIG. 1 illustrates an embodiment of a substrate 5 with a coating 35 ofmultiple bilayers 10. In an embodiment to produce the coated substrate 5shown in FIG. 1, the multilayer thin film coating method includesexposing substrate 5 to cationic molecules in a cationic mixture toproduce cationic layer 30 on substrate 5. The cationic mixture containscationic materials 20. In such an embodiment, the substrate 15 isnegatively charged or neutral. The cationic mixture includes an aqueoussolution of the cationic materials 20. The aqueous solution may beprepared by any suitable method. In embodiments, the aqueous solutionincludes the cationic materials 20 and water. In other embodiments,cationic materials 20 may be dissolved in a mixed solvent, in which oneof the solvents is water and the other solvent is miscible with water(e.g., water, methanol, and the like). The solution may also containcolloidal particles in combination with polymers or alone, if positivelycharged. Any suitable water may be used. In embodiments, the water isdeionized water. In some embodiments, the aqueous solution may includefrom about 0.05 wt. % cationic materials 20 to about 1.50 wt. % cationicmaterials 20, alternatively from about 0.01 wt. % cationic materials 20to about 1.00 wt. % cationic materials 20. In embodiments, the substrate5 may be exposed to the cationic mixture for any suitable period of timeto produce the cationic layer 30. In embodiments, the substrate 5 isexposed to the cationic mixture from about 1 second to about 20 minutes,alternatively from about 1 second to about 200 seconds, andalternatively from about 10 seconds to about 200 seconds. Without beinglimited by theory, the exposure time of substrate 5 to the cationicmixture and the concentration of cationic materials 20 in the cationicmixture affect the thickness of the cationic layer 30. For instance, thehigher the concentration of the cationic materials 20 and the longer theexposure time, the thicker the cationic layer 30 produced by themultilayer thin film coating method.

In embodiments, after formation of the cationic layer 30, multilayerthin film coating method includes removing substrate 5 with the producedcationic layer 30 from the cationic mixture and then exposing substrate5 with cationic layer 30 to anionic molecules in an anionic mixture toproduce anionic layer 25 on cationic layer 30 and thereby form bilayer10. The anionic mixture contains the layerable materials 15. Withoutbeing limited by theory, the positive cationic layer 30 attracts theanionic molecules to form the cationic-anionic pair of bilayer 10. Theanionic mixture includes an aqueous solution of the layerable materials15. The aqueous solution may be prepared by any suitable method. Inembodiments, the aqueous solution includes the layerable materials 15and water. Layerable materials 15 may also be dissolved in a mixedsolvent, in which one of the solvents is water and the other solvent ismiscible with water (e.g., ethanol, methanol, and the like).Combinations of anionic polymers and colloidal particles may be presentin the aqueous solution. Any suitable water may be used. In embodiments,the water is deionized water. In some embodiments, the aqueous solutionmay include from about 0.05 wt. % layerable materials 15 to about 1.50wt. % layerable materials 15, alternatively from about 0.01 wt. %layerable materials 15 to about 1.00 wt. % layerable materials 15. Inembodiments, the substrate 5 with cationic layer 30 may be exposed tothe anionic mixture for any suitable period of time to produce theanionic layer 25. In embodiments, the substrate 5 with cationic layer 30is exposed to the anionic mixture from about 1 second to about 20minutes, alternatively from about 1 second to about 200 seconds, andalternatively from about 10 seconds to about 200 seconds. Without beinglimited by theory, the exposure time of substrate 5 with cationic layer30 to the anionic mixture and the concentration of the layerablematerials 15 in the anionic mixture affect the thickness of the anioniclayer 25. For instance, the higher the concentration of the layerablematerials 15 and the longer the exposure time, the thicker the anioniclayer 25 produced by the multilayer thin film coating method. Substrate5 with bilayer 10 is then removed from the anionic mixture. Inembodiments, the exposure steps are repeated with substrate 5 havingbilayer 10 continuously exposed to the cationic mixture and then theanionic mixture to produce multiple bilayers 10 as shown in FIG. 1. Therepeated exposure to the cationic mixture and then the anionic mixturemay continue until the desired number of bilayers 10 is produced.

It is to be understood that the multilayer thin film coating method isnot limited to exposure to a cationic mixture followed by an anionicmixture. In embodiments in which substrate 5 is positively charged, themultilayer thin film coating method includes exposing substrate 5 to theanionic mixture followed by exposure to the cationic mixture. In suchembodiment (not illustrated), anionic layer 25 is deposited on substrate5 with cationic layer 30 deposited on anionic layer 25 to producebilayer 10 with the steps repeated until coating 35 has the desiredthickness. In embodiments in which substrate 5 has a neutral charge, themultilayer thin film coating method may include beginning with exposureto the cationic mixture followed by exposure to the anionic mixture ormay include beginning with exposure to the anionic mixture followed byexposure to the cationic mixture.

It is to be understood that coating 35 is not limited to one layerablematerial 15 but may include more than one layerable material 15 and/ormore than one cationic material 20. The different layerable materials 15may be disposed on the same anionic layer 25, alternating anionic layers25, or in layers of bilayers 10. The different cationic materials 20 maybe dispersed on the same cationic layer 30 or in alternating cationiclayers 30. For instance, in embodiments as illustrated in FIGS. 2-4,coating 35 includes two types of layerable materials 15, 15′ (i.e.,sodium montmorillonite is layerable material 15 and aluminum hydroxideis layerable material 15′). It is to be understood that substrate 5 isnot shown for illustrative purposes only in FIGS. 2-4. FIG. 2illustrates an embodiment in which layerable materials 15, 15′ are indifferent layers of bilayers 10. For instance, as shown in FIG. 2,layerable materials 15′ are deposited in the top bilayers 10 afterlayerable materials 15 are deposited on substrate 5 (not illustrated).FIG. 3 illustrates an embodiment in which coating 35 has layerablematerials 15, 15′ in alternating bilayers. It is to be understood thatcationic materials 20 are not shown for illustrative purposes only inFIG. 3. FIG. 4 illustrates an embodiment in which there are two types ofbilayers 10, comprised of particles (layerable materials 15, 15′) andcationic materials 20, 20′ (e.g., polymers).

In some embodiments, the multilayer thin film coating method includesrinsing substrate 5 between each exposure step (i.e., step of exposingto cationic mixture or step of exposing to anionic mixture). Forinstance, after substrate 5 is removed from exposure to the cationicmixture, substrate 5 with cationic layer 30 is rinsed and then exposedto an anionic mixture. After exposure to the anionic mixture, substrate5 with bilayer 10 is rinsed before exposure to the same or anothercationic mixture. The rinsing is accomplished by any rinsing liquidsuitable for removing all or a portion of ionic liquid from substrate 5and any layer. In embodiments, the rinsing liquid includes deionizedwater, methanol, or any combinations thereof. In an embodiment, therinsing liquid is deionized water. Substrate 5 may be rinsed for anysuitable period of time to remove all or a portion of the ionic liquid.In an embodiment, substrate 5 is rinsed for a period of time from about5 seconds to about 5 minutes. In some embodiments, substrate 5 is rinsedafter a portion of the exposure steps.

In embodiments, the multilayer thin film coating method includes dryingsubstrate 5 between each exposure step (i.e., step of exposing tocationic mixture or step of exposing to anionic mixture). For instance,after substrate 5 is removed from exposure to the cationic mixture,substrate 5 with cationic layer 30 is dried and then exposed to ananionic mixture. After exposure to the anionic mixture, substrate 5 withbilayer 10 is dried before exposure to the same or another cationicmixture. The drying is accomplished by applying a drying gas tosubstrate 5. The drying gas may include any gas suitable for removingall or a portion of liquid from substrate 5. In embodiments, the dryinggas includes air, nitrogen, or any combinations thereof. In anembodiment, the drying gas is air. In some embodiments, the air isfiltered air. Substrate 5 may be dried for any suitable period of timeto remove all or a portion of the liquid. In an embodiment, substrate 5is dried for a period of time from about 5 seconds to about 500 seconds.In an embodiment in which substrate 5 is rinsed after an exposure step,substrate 5 is dried after rinsing and before exposure to the nextexposure step. In alternative embodiments, drying includes applying aheat source to substrate 5. For instance, in an embodiment, substrate 5is disposed in an oven for a time sufficient to remove all or a portionof the liquid. In alternative embodiments, drying includes squeezingsubstrate 5 to wring the liquid out. In some embodiments, drying is notperformed until all layers have been deposited, as a final step beforeuse.

In some embodiments (not illustrated), additives may be added tosubstrate 5 in coating 35. In embodiments, the additives may be mixed inanionic mixtures with layerable materials 15. In other embodiments, theadditives are disposed in anionic mixtures that do not include layerablematerials 15. In some embodiments, coating 35 has a layer or layers ofadditives. In embodiments, the additives are anionic materials. Theadditives may be used for any desirable purpose. For instance, additivesmay be used for protection of substrate 5 against ultraviolet light orfor abrasion resistance. For ultraviolet light protection, anynegatively charged material suitable for protection against ultravioletlight and for use in coating 35 may be used. In an embodiment, examplesof suitable additives for ultraviolet protection include titaniumdioxide, or any combinations thereof. In embodiments, the additive istitanium dioxide. For abrasion resistance, any additive suitable forabrasion resistance and for use in coating 35 may be used. Inembodiments, examples of suitable additives for abrasion resistanceinclude crosslinkers. Examples of crosslinkers include glutaraldehyde,bromoalkanes, or any combinations thereof. The crosslinkers may be usedto crosslink the anionic layers 25 and/or cationic layers 30. In anembodiment, substrate 5 with bilayer 10 is exposed to additives in ananionic mixture in the last exposure step.

In some embodiments, the pH of anionic and/or cationic solution isadjusted. Without being limited by theory, reducing the pH of thecationic solution reduces growth of coating 35. Further, without beinglimited by theory, the coating 35 growth may be reduced because thecationic solution may have a high charge density at lowered pH values,which may cause the polymer backbone to repel itself into a flattenedstate. In some embodiments, the pH is increased to increase the coating35 growth and produce a thicker coating 35. Without being limited bytheory, a lower charge density in the cationic mixture provides anincreased coiled polymer. The pH may be adjusted by any suitable meanssuch as by adding an acid or base.

The exposure steps in the anionic and cationic mixtures may occur at anysuitable temperature. In an embodiment, the exposure steps occur atambient temperatures. In some embodiments, the fire retardant coating isoptically transparent.

Without being limited by theory, the fire retardant coating covers theinternal walls of the pores of the substrate without blocking the pores.For instance, in an embodiment in which the substrate is a fabriccomprising threads, the multilayer thin film coating method mayindividually coat each thread with the fire retardant coating. Further,without being limited by theory, coating each thread provides flameretardancy to the substrate but allows the threads to remain soft andflexible.

To further illustrate various illustrative embodiments of the presentinvention, the following examples are provided.

EXAMPLES Example 1

Preparation of Deposition Mixtures. Cationic deposition solutions wereprepared by dissolving 0.1 wt. % branched polyethylenimine, with amolecular weight of 25,000 g/mol (commercially available from Aldrich ofMilwaukee, Wis., into 18.2 MΩ deionized water from a Direct-QTM 5Ultrapure Water System (commercially available from Millipore ofBillerica, Mass.). The unadjusted pH of this solution was 10.3, but thisvalue was adjusted to 7 and 10 by adding 1M hydrochloric acid(36.5-38.0% HCl available from Mallinckrodt Chemicals of Phillipsburg,N.J.). Sodium montmorillonite (MMT) (Cloisite® Na⁺ a trademark ofSouthern Clay Products, Inc. of Gonzales, Tex.) was exfoliated by addingit to deionized water (0.2 or 1.0 wt. %) and slowly rolling for 24 h, toproduce the anionic deposition mixtures. MMT had a cationic exchangecapacity of 0.926 meq/g and a negative surface charge in deionizedwater. Individual platelets had a density of 2.86 g/cm³, with a planardimension of 10-1,000 nm (average was around 200 nm) and a thickness of1 nm. The pH was measured with an Accumet® Basic AB15 pH meter(commercially available from Fisher Scientific Company of Pittsburgh,Pa.).

Substrates. Single-side-polished silicon wafers (commercially availablefrom University Wafer of South Boston, Mass.) were used as depositionsubstrates for films characterized by ellipsometry and AFM. PolishedTi/Au crystals with a resonance frequency of 5 MHz were purchased fromMaxtek, Inc. of Cypress, Calif. and used as deposition substrates forquartz crystal microbalance characterization. TEM imaging of these filmsused 125 μm polystyrene (PS) film (commercially available fromGoodfellow of Oakdale, Pa.) as the substrate for deposition. Prior todeposition, silicon wafers were rinsed with acetone, then deionizedwater, and finally dried with filtered air. In the case of PSsubstrates, the film was rinsed with methanol and deionized water, anddried with air. The clean PS substrates were then corona-treated with aBD-20C Corona Treater (commercially available from Electro-TechnicProducts Inc. of Chicago, Ill.) for 2 minutes. Corona treatment oxidizesthe PS film surface and creates a negative surface charge, whichimproves adhesion of the first BPEI layer. Scoured and bleachedplain-woven cotton fabric, that was coated and tested for thermalstability, was supplied by the United States Department of Agriculture(USDA) Southern Regional Research Center (SRRC, New Orleans, La.). Thefabric was a balanced weave with approximately 80 threads per inch inboth the warp and fill direction, with a weight of 119 g/m². The controlfabric was treated by laundering through a cold water cycle, with nodetergent, in a standard commercial high-efficiency clothes washer anddried for approximately 30 minutes in a commercial electric clothesdryer (commercially available from Whirlpool Corporation of BentonHarbor, Mich.). The wet processing of the control fabric was intended toeliminate any changes in physical construction of the fabric due to thewet processing of the fabric during the LbL deposition and was then usedas the uncoated fabric in all tests.

Layer-by-Layer Deposition. All films were assembled on a givensubstrate. Each substrate was dipped into the ionic depositionsolutions, alternating between the BPEI (cationic) and MMT (anionic),with each cycle corresponding to one bilayer. The first dip into eachmixture was for five minutes, beginning with the cationic solution.Subsequent dips were for two minutes each. Every dip was followed byrinsing with deionized water and drying with a stream of filtered airfor 30 seconds each. In the case of the fabrics, the drying stepinvolved wringing the water out instead of air-drying. After achievingthe desired number of bilayers, the coated wafers were dried withfiltered air, whereas the fabrics were dried in an 80° C. oven for 2hours.

Film Growth Characterization. Film thickness was measured on siliconwafers using a PhE-101 Discrete Wavelength Ellipsometer (commerciallyavailable from Microphotonics of Allentown, Pa.). The HeNe laser (632.8nm) was set at an incidence angle of 65°. A Maxtek Research QuartzCrystal Microbalance (QCM) from Infinicon of East Syracuse, N.Y., with afrequency range of 3.8-6 MHz, was used in conjunction with 5 MHz quartzcrystals to measure the weight per deposited layer. The crystal, in itsholder, was dipped alternately into the positively andnegatively-charged solutions. Between each dip, the crystal was rinsed,dried, and left on the microbalance for five minutes to stabilize.Cross-sections of the clay-polymer assemblies were imaged with a JEOL1200 EX TEM (commercially available from Mitaka of Tokyo, Japan),operated at 110 kV. Samples were prepared for imaging by embedding apiece of PS supporting the LbL film in epoxy and sectioning it with amicrotome equipped with a diamond knife. Surface structures were imagedwith a Nanosurf EasyScan 2 Atomic Force Microscope (AFM) (commerciallyavailable from Nanoscience Instruments, Inc. of Phoenix, Ariz.). AFMimages were gathered in tapping mode with a XYNCHR cantilever tip. ABruker-AXS D8 Advanced Bragg-Brentano X-ray Powder Diffractometer (CuKα, λ=1.541 Å) (commercially available from BRUKER AXS Inc. of Madison,Wis.) was used for both powder diffraction and glancing angle XRD.Contact angle measurements were done using a CAM 200 Optical ContactAngle Meter (commercially available from KSV Instruments Ltd. ofHelsinki, Finland).

Thermal, Flammability, and Combustibility Testing. All tests wereconducted in triplicate for each system to obtain the reported averages.The thermal stability of uncoated and coated fabrics was measured in aQ50 Thermogravimetric Analyzer (commercially available from TAInstruments of New Castle, Del.). Each sample was approximately 20 mgand was tested in an air atmosphere, from room temperature to 600° C.,with a heating rate of 20° C./min. Vertical flame testing was performedon 3×12 in. sections of uncoated and coated fabrics according to ASTMD6413. An Automatic Vertical Flammability Cabinet, model VC-2(commercially available from Govmark of Farmingdale, N.Y.), was used toconduct this testing. The Bunsen burner flame, 19 mm below the fabricsample, was applied for twelve seconds, after which the after-flame andafter-glow times were measured. Microscale combustibility experimentswere carried out in a Govmark MCC-1 Microscale Combustion Calorimeter.The specimens were first kept at 100° C. for 5 min to remove adsorbedmoisture, and then heated up to 700° C. at a heating rate of 1° C./sec,in a stream of nitrogen flowing at 80 cm³/min. The pyrolysis volatilesreleased from the thermal degradation of the sample into the nitrogengas stream were mixed with a 20 cm³/min stream of pure oxygen prior toentering a 1000° C. combustion furnace. Three samples weighing about 4.3mg were tested for each system.

Analysis of Fabric. Surface images of control and coated fabrics, aswell as afterburn chars (after direct exposure to flame), were acquiredwith a Quanta 600 FE-SEM (commercially available from FEI Company ofHillsboro, Oreg.). Physical properties of the fabric were tested atUSDA-SRRC using ASTM and AATCC (American Association of Textile Chemistsand Colorists) Standards. ASTM D 3775 was used to determine the fabriccount on the fabric sample, counting the number of yarns in the warp andfill directions at five different locations to determine the averagenumber of yarns per inch. ASTM D 1424 was used to determine the fabric'sresistance to tearing. This test was carried out using the Elmendorffalling pendulum apparatus (commercially available from SDL Atlas ofStockport, UK). Two clamps secured the sample and a slit was cut downthe center before a pendulum action attempted to tear the fabric.Control samples were tested five times and coated samples were testedthree times due to insufficient material to allow for five testspecimens. ASTM D 5035 was used to determine the breaking force andpercent of apparent elongation. A sample piece of fabric was placed in aconstant-rate-of-extension tensile testing machine, and a force wasapplied until the sample broke (commercially available from InstronCorporation of Norwood, Mass.). As with the Elmendorf test, controlsamples were tested five times, and coated samples were tested threetimes. To determine water-wicking ability, the AATCC Committee RA63proposed test method for wicking was employed. A 25 mm×175 mm strip offabric was placed in a beaker with water, and the time it took the waterto climb 2 cm vertically was measured. All fabrics were pre-conditionedat 21° C. and 65% RH (according to ASTM D 1776) for 48 hours beforetesting.

Results and Discussion

Growth of Clay/Polymer Assemblies. The influence of pH and concentrationof the deposition mixtures on the growth of the thin films was evaluatedby ellipsometry. Four different thin film recipes, BPEI pH 7 and 10,with MMT at 0.2 wt. % and 1 wt. %, were used to prepare the films withthe growth shown in FIG. 5. FIG. 5 shows film thickness as a function ofthe number of bilayers deposited, for a series of LbL assemblies madewith varying pH of the BPEI solution and concentration of the MMTmixture. MMT was used at its unadjusted pH of 9.8. All four systems grewlinearly as a function of BPEI-MMT bilayers deposited. The filmthicknesses were very similar for films made with the same pH BPEIsolution, regardless of variation in clay concentration. Differencesobserved between high and low pH systems were due to the differentdegrees of charge density of the weak polyelectrolyte BPEI. When thisweak polyelectrolyte was highly charged, the polymer chains adopted aflat conformation, whereas at low charge density, the polymer had a morecoiled and bulky conformation. In order to better understand the growthprocess, a QCM was used to measure the weight increase with thedeposition of each individual layer.

FIG. 6 shows the QCM data for the four different recipes describedabove. FIG. 6 shows film mass as a function of individually depositedclay and polymer layers for four different BPEI/MMT systems. In allcases, odd layers are BPEI and even ones are MMT. There was not muchdifference observed in mass per layer of the films made with pH 7 BPEIand two different concentrations of MMT mixture (0.2 and 1 wt. %), butthe films made with pH 10 BPEI and two concentrations of MMT show asignificant difference in unit mass. The amount of BPEI deposited foreach layer was similar between the films made with the same pH, but BPEIat pH 7 deposited less in each layer than BPEI pH 10 (about one-thirdthe amount). Table 1 summarizes the BPEI and MMT compositions that werecalculated for each film. The films made with 1 wt. % MMT and BPEI atdifferent pH values had higher MMT content than films made with 0.2 wt.% MMT. In all four film recipes, it was believed that film thickness wasinfluenced primarily by the pH of the BPEI solution and only slightly bythe concentration of clay. Film weight was quite different, with MMTconcentration of the deposition mixture becoming significant at thehigher pH of BPEI. This may be explained by the following models. WhenBPEI had a higher charge density at low pH, it lies flatter on thecharged substrate due to self-repulsion, and the clay platelets may onlylay parallel to the substrate, covering the topmost surface. In thiscase, films made with 1 wt. % MMT mixtures achieved slightly bettercoverage per deposition than films made with 0.2 wt. % MMT, resulting insimilar thicknesses and weights for the two films. When BPEI had a lowercharge density (at pH 10), it was more coiled and entangled, thuscreating thicker films as it was deposited. This thicker layer allowedmore clay platelets to deposit in the pockets between coils and tangles.In this scenario, a higher concentration of MMT (1 wt. %) may providefor more loading of the BPEI pockets during each deposition step thanthe more dilute mixture (0.2 wt. % MMT).

TABLE 1 Film composition of BPEI/MMT recipes. LbL system BPEI wt % MMTwt % BPEI (pH 10)/0.2 wt % MMT 22 ± 6 78 ± 13 BPEI (pH 7)/0.2 wt % MMT 28 ± 10 72 ± 25 BPEI (pH 10)/1 wt % MMT 17 ± 6 83 ± 12 BPEI (pH 7)/1 wt% MMT 13 ± 6 87 ± 18

Tapping mode AFM was used to characterize the surfaces of 30 BLMMT-composite films made with high and low pH of BPEI. Theroot-mean-square (rms) of the area roughness (using a 20 μm square area)for the BPEI pH 7/1 wt. % MMT film was 38 nm, while it was 62 nm for theBPEI pH 10/1 wt. % MMT film, which suggested that the surface wascovered by clay platelets with a largest dimension oriented parallel tothe surface of the silicon substrate. Because of the differentmorphology of BPEI at high and low charge densities, the surface wasrougher for films made with pH 10 BPEI. A 40 BL film was made with BPEIpH 10/0.2 wt. % MMT. The film was deposited on polystyrene substrates tofacilitate sectioning. All surfaces were well covered by the depositedMMT platelets.

Flame Resistance of Fabric. Cotton fabric was coated with 5 and 20bilayers of BPEI/MMT, using the four different recipes described in theprevious section describing thin film growth. The coating weight wasdetermined by weighing 12 by 15 in. samples of fabric before and aftercoating. All samples were weighed only after oven-drying at 80° C. for 2hours to remove moisture. Weight added to the fabric by each coatingsystem is shown in Table 2 as a percentage of the uncoated weight. Theweight gain from coating on fabric does not correlate well to the weightgain measured by QCM for the films assembled on a quartz crystal. At 5BL, fabric coated using BPEI at pH 10 was heavier than fabric coatedusing pH 7 BPEI, but at 20 BL the fabric weight gain was greater with pH7 BPEI. This may be linked to differences in adhesion and substrategeometry.

TABLE 2 Weight added by coating fabrics, and residue amounts after heattreatment. 500° C. 600° C. Add-on (%) residue (%) residue (%) Sample 5BL 20 BL 5 BL 20 BL 5 BL 20 BL Control 1.77^(b) 0.30^(b) BPEI pH10/0.2%2.05 2.31 9.12 11.70 1.29 2.09 MMT BPEI pH 7/0.2% 0.97 2.89 7.00 10.391.17 3.28 MMT BPEI pH10/1% MMT 2.23 4.06 11.26 12.16 1.70 2.82 BPEI pH7/1% MMT 1.82 4.41 9.33 13.02 1.47 4.72 ^(a)Residue values obtained fromTGA testing under air atmosphere. ^(b)The residue weight percent ofuncoated fabric.

Two coatings were prepared using a 1 MMT mixture with BPEI at high andlow pH. All of the individual cotton fibers were easily discerned forthe 20 BL coating made with BPEI at pH 10. The same coating appliedusing BPEI at pH 7 appeared thicker and stickier, actually bridgingmultiple fibers.

FIGS. 7a ) and 7 b) show TGA results of four coating recipes at 5 (FIG.7a )) and 20 BL (FIG. 7b )). Weight loss as a function of temperaturefor cotton fabrics coated with 5 bilayers is shown in FIG. 7a ), and 20bilayers is shown in FIG. 7b ) with both Figures having 0.1 wt. % BPEI(pH 10 and 7) with 0.2 and 1 wt. % MMT. The results were obtained usingTGA at a heating rate of 20° C./min under an air atmosphere. At 500° C.,under an air atmosphere, the uncoated control fabric left less than 1.8wt. % residue, as shown in FIGS. 7a ) and 7 b). With the addition of 2wt. % for a 5 BL coating and 4 wt. % for a 20 BL coating, residue weightpercentages for the coated fabrics were one order of magnitude higherthan the control. The residue amounts for the control fabric and eachcoated fabric were summarized in Table 2. At the final stage of thetesting, there was essentially no char left from the control fabric, butthere was a significant amount of residue left from 20 BL-coatedfabrics. The mass of the residue from a coated fabric clearlydemonstrated that there was preservation of cotton during burning,because some residues were greater than the mass of the coating itself(see add-on % in Table 2). The amount of charred cotton in the residuewas probably higher than the mass difference between residue and thecoating by itself (in all cases), because at least a fraction of theBPEI in the coating was degraded during heating (pure BPEI completelydecomposes below 650° C.). There was a direct correlation between addedcoating weight (Table 2) and residue generated in the TGA. Additionally,the better surface coverage by the pH 7 BPEI system at 20 BL resulted in10% greater coating weight, but 67% greater char at 600° C.

An equivalent set of coated fabric samples was put through verticalflame testing (ASTM D6413). Time to ignition did not increase uponcoating the fabric, but a brighter and more vigorous flame was observedon the control fabric compared to the coated fabrics at 5 seconds afterignition. The flame on the coated fabric was not very vigorous.Additionally, more glow was seen on the control fabric after the flamewas removed. The control and eight different coated fabrics showedsimilar after-flame times (i.e., time that fire was observed on samplesafter direct flame removed), but the afterglow times for coated fabricswere 9 seconds less than for the uncoated fabric. Table 3 summarizesafter-flame and after-glow times for each recipe. After burning, nocontrol fabric was left on the sample holder, but all four 20 BL-coatedfabrics left significant residues. The residues from 20 BL-coatedfabrics were heavier and preserved the fabric structure better than theresidues from fabrics coated with only 5 BL.

All fabrics were imaged by scanning electron microscopy, before andafter flame testing, to evaluate the surface morphology and fabricstructure. The control fabric left only ash after flame exposure, sothese ashes were for imaging, whereas coated fabric images were morerepresentative from the center of the charred remains. The fiber surfacein the control fabric appeared very clean and smooth compared to thecoated fabrics. Small MMT aggregates were seen on the fibers of thecoated fabrics that were likely the result of inefficient rinsing offabric between layers. Each fiber of the fabric was at least partially,if not completely, covered by the clay coating. After flame testing, theash from the uncoated fabric and the residue from coated fabric wereimaged under the same magnification. It was viewed that the ashes of theuncoated cotton fabric no longer had the same fabric structure and shapeof the original fibers. Broken pieces and holes in the fiber strandsillustrated the complete destruction that occurred during burning ofuncoated cotton. It was surprising that with only 5 BL, the fabricstructure was maintained, and the fibers were relatively intact. It wasbelieved that during burning at high temperature, the MMT plateletsfused together to some extent, which accounted for not seeing aggregatedMMT or the edges of the platelets after burning, but rather largecontinuous pieces of coating instead. The dimensions of the weavestructure in uncoated and coated fabrics were identical, which meansthat the LbL coating process did not alter the fabric dimensions. Afterburning, ash remaining from the uncoated fabric did not show the weavestructure anymore, but the residue from coated fabrics retained theweave structure, especially the 20 BL, BPEI pH 7/1 wt. % MMT-coatedfabric. Even the width of individual yarns is similar to the widthbefore burning for this sample. The 5 BL (BPEI pH 7/1 wt. % MMT)-coatedfabric also retained its weave structure, although the threads shrankafter flame testing, leaving gaps between the yarns. Despite using thesame concentration of clay deposition mixture (1 wt. % MMT), the weavestructure of the residue from 20 BL-coated fabric made using pH 10 BPEIhad larger gaps between yarns as compared to the fabric coated (20 BL)using BPEI pH 7. This was an expected result due to the smaller add-onpercentage of the BPEI pH 10 coating, as well as to the greater surfacecoverage achieved by the coating when highly charged pH 7 BPEI is used.

The XRD pattern in FIG. 8 provided additional evidence of the coating ofthe fabric. In FIG. 8, the low-angle peak at 7.8° for neat MMT clay wasderived from a basal spacing of 11.4 Å, which was the periodic distancefrom platelet to platelet. X-ray diffraction patterns were for neat MMT,for 20 BL BPEI pH 7/1 wt. % MMT coated fabric, before and after burning,and for the control fabric. On the fabric coated with BPEI pH 7/1 wt. %MMT, the peak was shifted to 6.4°, suggesting that even on the non-flatfiber surface the clay may be deposited in an orderly orientation; thebasal spacing was increased to 13.7 Å because of intercalation withBPEI. After vertical flame testing, the residue from coated fabric wasalso scanned by XRD, finding that the basal spacing decreased from 13.7to 12.7 Å, which suggested that the intercalated BPEI might decompose orbe ablated during the burning process, resulting in a reduction of thebasal spacing of MMT. The positions of the low-angle MMT peak in scans(data not shown) of fabric coated with BPEI pH 10/1 wt. % MMT (beforeand after flame test) showed no significant difference between the tworecipes.

Another tool for assessing the fire behavior of a small (mg) sample wasthe microscale combustion calorimeter (MCC). The MCC simulated theburning process by using anaerobic pyrolysis and a subsequent reactionof the volatile pyrolysis products with oxygen under high temperaturesto simulate surface gasification and flaming combustion. Both heatrelease rate and temperature as a function of time at constant heatingrate were measured during the test. Key parameters coming from the MCCtest included temperature at maximum heat release rate (Tp), specificheat release rate (HRR in W/g) that was obtained by dividing the heatrelease rate at each point in time by the initial sample mass, and totalheat release (THR in kJ/g) from combustion of the fuel gases per unitmass of initial sample (obtained by time-integration of HRR over theentire test). Residue was calculated by weighing the sample before andafter the test. A derived quantity, the heat release capacity (HRC inJ/g K) was obtained by dividing the maximum value of the specific heatrelease rate by the heating rate during the test. HRC was a molecularlevel flammability parameter that was a good predictor of flameresistance and fire behavior when only research quantities wereavailable for testing. Reproducibility of the test for homogeneoussamples was about ±8%.

MCC data for the coated fabric samples was summarized in Table 3. Allresidues from coated fabrics tested at 700° C. under nitrogen atmospherewere higher than those from uncoated fabric. The residue did not comeonly from the coating (see add-on wt. % in Table 2), but the fabricitself was preserved (1-5 wt. %) when coated with various recipes. Theseresults suggested that clay surrounded each fiber and acted as aprotective barrier capable of promoting char formation during thepyrolysis of the fabric. An increase in charring induced a decrease inthe amount and rate of combustible volatile release, resulting in lowerflammability (as evidenced by lower THR and HRC values in the MCC). Themaximum reduction in THR (20%) and HRC (15%) as compared to the controlwas observed in the fabric coated with 5 BL of BPEI pH10/1 wt. % MMT.Increasing the number of bilayers up to 20 for the same sample did notappear to produce any significant variation in the MCC data. Thissuggested that a 5 BL coating may be sufficient for generating aneffective fire barrier on the textile. An increase in Tp was alsoobserved in all coated fabrics, which was likely due to the formation ofa low permeability barrier that delayed the release of combustiblevolatiles.

TABLE 3 Microscale combustion calorimeter results for various coatedfabrics. Residue (%) HRC (J/g K) THR (kJ/g) Tp (° C.) Sample 5 BL 20 BL5 BL 20 BL 5 BL 20 BL 5 BL 20 BL Control 2.88 ± 0.40 273.67 ± 25.3811.63 ± 0.21 369 ± 0.58 BPEI pH10/0.2% MMT 6.38 ± 1.50 7.48 ± 0.50254.33 ± 25.01 250.33 ± 14.50 11.23 ± 0.25 11.10 ± 0.36 374 ± 0.58 376 ±2.65 BPEI pH 7/0.2% MMT 6.75 ± 0.60 6.74 ± 0.20 260.33 ± 4.04  286.33 ±8.51  11.17 ± 0.40 11.90 ± 0.36 376 ± 2.00 369 ± 0.58 BPEI pH10/1% MMT10.52 ± 0.30  10.49 ± 0.50  220.00 ± 6.08  221.30 ± 7.57   9.87 ± 0.3110.23 ± 0.06 382 ± 0.58 380 ± 0.58 BPEI pH 7/1% MMT 8.37 ± 0.50 10.54 ±0.30  251.30 ± 10.02 240.30 ± 11.37 10.73 ± 0.25 10.70 ± 0.50 379 ± 1.00377 ± 2.65

Physical Properties of Fabric. There was no difference in appearancebetween coated and uncoated fabric. Even tactile assessment of thefabric by touch of hand, was the same for all coated and uncoatedsamples tested. As a result of this similarity, some measurements wereneeded to distinguish between the coated and uncoated fabric. In manycases, the addition of a flame retardant resulted in loss of strength orthe degradation of other fabric properties (e.g., moisture wicking), soit was important to know if this coating technology changed theproperties of the fabric. Fabric count, tear and tensile strength, andwicking behavior of coated fabrics were evaluated in comparison withcontrol fabric.

Fabric count was determined by following the ASTM D 3775 standardmethod. Yarn number in the warp and fill directions of fabric wascounted on a 25×25 mm area of the fabric. Five randomly selected areasfrom each coated fabric were used to determine the average fabric count.These counts are summarized in Table 4 where the yarn numbers of 5BL-coated fabrics in both directions are shown to be only 1.2% differentfrom the control fabric. For the 20 BL-coated fabrics, the yarn numberis less than 2.5% different in warp direction, while in fill directionthere is less than a 5% difference. Therefore, the coating of polymerand clay layers on the fabric did not significantly alter the fabric'sphysical structure. Wet processing of cotton fabrics with traditionaltextile finishes often causes shrinkage and compaction in the yarns,resulting in more yarns per inch and affecting the comparison ofphysical properties of the treated fabrics to control materials.

TABLE 4 Fabric counts of uncoated and coated fabrics. BL Sample numberWarp Fill Control 79 78 BPEI pH10/0.2% MMT 5 78 79 20 81 81 BPEI pH7/0.2% MMT 5 78 78 20 80 82 BPEI pH 10/1% MMT 5 80 78 20 78 79 BPEI pH7/1% MMT 5 79 79 20 77 79

The Elmendorf tearing test, which used a falling pendulum to determinethe amount of force required to tear the fabric (ASTM D 1424), was usedto evaluate tear strength. A strip tensile strength test was used todetermine the maximum force that can be applied to a material (sampledas a strip) until it fractured (ASTM D 5035). Additionally, the striptest measured the apparent elongation of the fabric. The Elmendorf andtensile tests showed similar results, which were summarized in Table 5.The warp direction for the coated fabrics exhibited improvement in bothtearing and breaking strength when compared to the control fabric, whilethe fill direction showed a general decrease in strength. The elongationresults had slight directionality as well. The warp direction showed adecrease in elongation, while the fill direction showed an increase. Allof these properties were within 10% of the uncoated fabric, so the datadid not reveal a clear connection between coating and strengthproperties. The sporadic nature of the results as well as the incoherentpattern suggested that the results were not based on a change in fiberstructure due to the coating but rather were within the range ofstrength and elongation for the uncoated fabrics. In other words, thecoating neither greatly improved nor harmed the fabric's mechanicalstrength. It was therefore assumed that there was not a significantchange to the structure of the fibers during the coating process, whichwas an improvement relative to traditional textile finishing.

TABLE 5 Tearing force and tensile breaking force of uncoated and coatedfabrics. Tearing Breaking Elongation BL Force (lbs) Force (lbs) (%)Sample number Warp Fill Warp Fill Warp Fill Control 2.11 2.02 66.3069.34 19.5 30.7 BPEI pH10/0.2% 5 2.26 1.99 72.92 68.09 15.7 38.5 MMT 202.25 2.02 67.23 63.66 16.9 36.4 BPEI pH 7/0.2% 5 2.24 2.12 80.33 65.8814.7 36.8 MMT 20 2.22 2.05 75.29 66.13 14.7 36.3 BPEI pH10/1% 5 2.211.86 80.11 61.54 12.1 30.1 MMT 20 2.25 1.80 78.58 73.50 13.5 31.2 BPEIpH 7/1% 5 2.29 2.01 71.35 66.43 12.8 31.1 MMT 20 2.04 1.87 68.76 63.2314.5 30.8

The AATCC Committee RA63 proposed test method for wicking was used totest the transfer of water through the various fabric samples. Moststandard fabrics absorb water through capillary action, using the gapsbetween warp and fill yarns as small capillaries, causing them to absorba comparatively large amount of water. The wicking test measured thetime it takes water to travel up a piece of fabric in an Erlenmeyerflask or beaker. Shorter wicking times (i.e., faster movement of waterup the test strip) indicated better wicking ability. The wickingdistance was 20 mm, and wicking rates were calculated by dividing thewicking distances by the average wicking times. Wicking rates in thewarp and fill directions of each fabric were summarized in Table 6. Forall the coated fabrics, both warp and fill wicking rates were muchslower (by a factor of 2-3) than the control fabric, indicating thattheir ability to absorb and transport water was not as great as thecontrol. This was not so surprising, considering the outermost claylayer had been analyzed using ab initio molecular dynamics where it wasconcluded that its tetrahedral surface (i.e., the oxygen plane, whichwas the widest dimension in MMT surface) may be considered hydrophobic.In addition, contact angle results were 72° for a coating of BPEI pH 7/1wt. % MMT on a Si wafer, and 74° for BPEI pH 10/1 wt. % MMT, suggestingthat the MMT-covered surface was more hydrophobic since both contactangles were larger than the 38° measured for a bare Si wafer. Among thefour different systems of fabric coatings, the ones involving BPEI pH 7have slower wicking rates than those made using BPEI pH 10, whichsuggested that it was harder for water to be transported through pH 7BPEI coated fabrics. This behavior may be explained as caused by the MMTplatelets lying parallel to the fiber surface during deposition, withhighly charged BPEI at pH 7 packing the platelets especially tightly.Such an arrangement of clay platelets, which were slightly hydrophobic,provide excellent coverage and sealing of fiber surfaces, thusinterfering with the moisture transport both along and through thefiber.

TABLE 6 Vertical wicking rate of fabrics. Wicking Rate BL (mm/s) Samplenumber Warp Fill Control 2.50 2.61 BPEI pH10/0.2% MMT 5 1.25 0.91 201.22 1.00 BPEI pH 7/0.2% MMT 5 0.72 0.48 20 0.82 0.67 BPEI pH 10/1% MMT5 2.00 0.79 20 1.22 0.97 BPEI pH 7/1% MMT 5 0.81 0.44 20 0.86 0.61

Conclusions

This study focused on various BPEI/MMT thin film assemblies, with thegoal of developing a flame-retardant coating system for cotton fabrics.Films assembled with high and low pH polyethylenimine and 1 and 0.2 wt.% clay suspensions all showed linear growth as a function of the numberof bilayers deposited. Higher BPEI pH resulted in much thickerassemblies due to lower charge density. With respect to clay, using ahigher concentration resulted in slightly thicker films. Theselayer-by-layer assembled coatings were applied to cotton fabric toevaluate flammability. Flame-retardant properties of 5 and 20 BLcoatings on fabric were tested with TGA, vertical flame testing, andmicrocombustion calorimetry. A 6 to 11% residue was left over fromcoated fabric after heat treatment at 500° C. under air atmosphere,whereas the control fabric completely combusted. This level of charringis significant, because the coating contributed only 1 to 4 wt. % to thefabric (depending on recipe and number of layers) prior to burning.During actual burning in the vertical flame test, afterglow time wassignificantly reduced for the coated fabrics. The weave structure of thefabric, as observed in SEM images, was well preserved relatively in thechars from coated fabrics, whereas the scant ashes from the controlfabric showed little structure. SEM also revealed that each individualyarn was protected by the sheath-like coating. Additionally,microcalorimeter testing revealed lower heat release for coated fabrics,suggesting that fewer combustible volatiles were generated. The physicalproperties of the fabrics did not show great differences between controland coated, suggesting that the coating did not adversely affect thedesirable properties of the fabric itself. The relative simplicity ofthe layer-by-layer process provides a convenient method for impartingflame resistance to fabric using relatively benign ingredients. Inaddition to clays, other types of flame-retardant particles and polymersmay be considered for use in these types of coatings.

Example 2

Southern Clay Products, Inc. (Gonzales, Tex.) supplied the naturalsodium montmorillonite (MMT) (Cloisite® Na+a trademark of Southern ClayProducts, Inc.) used in this study. MMT had a cationic exchange capacityof 0.926 meq/g and a negative surface charge in deionized water.Individual platelets had a density of 2.86 g/cm³, diameter of 10-1000 nm(most have d>200 nm) and a thickness of 1 nm. Cationic polyacrylamide(Superfloc® C-491, a trademark of American Cyanamid Company) wasprovided by CYTEC (West Paterson, N.J.).

This was a copolymer containing 5 mol % of positively-charged repeatunits. Cationic poly (diallyldimethylammonium chloride) (PDDA) andbranched polyethylenimine (BPEI) (M_(w)=25,000 g/mol and M_(n)=10,000g/mol) were purchased from Aldrich (St. Louis, Mo.). Polystyrene (PS)film, with a thickness of 250 μm, was purchased from Goodfellow (UK) andused as the substrate for transmission electron microscopy. Single sidepolished (1 0 0), 500 μm thick silicon wafers (University Wafer, SouthBoston, Mass.) and fused quartz glass slides (Structure Probe Inc., WestChester, Pa.) were used as substrates for film growth characterized byellipsometry and UV-Vis, respectively. Polished Ti/Au crystals with aresonance frequency of 5 MHz were purchased from Maxtek, Inc (Cypress,Calif.) and used as the deposition substrates for quartz crystalmicrobalance characterization. Open-cell Polyurethane (PU) foam wasprovided by the NIST (Gaithersburg, Md.) and virgin cotton fabrics weresupplied by the USDA Southern Regional Research Center (New Orleans,La.).

Film Preparation

Aqueous solutions of branched polyethylenimine (0.1 wt. % in deionizedwater) were prepared by rolling for 24 hours. Prior to deposition, eachBPEI solution's pH was altered using 1M HCl. Anionic suspensions of MMT(0.2 wt. % in deionized water) were prepared by rolling for 24 h. Fordeposition PS, substrates were rinsed with deionized water, methanol,and again with water before finally being dried with filtered air. Thesesubstrates were then corona treated using a BD-20C Corona Treater(Electro-Technic Products, Inc., Chicago, Ill.), creating a negativesurface charge. For deposition onto silicon wafers, the substrates weresonicated for 30 minutes, and a piranha treatment was performed. Thesesubstrates were then rinsed with deionized water, acetone, and againwith water before finally being dried with filtered air. Forpolyurethane foams and cotton fabrics, the substrates were just rinsedand cleaned with deionized water. Each substrate was then dipped in thePEI solution for 5 min., rinsed with deionized water, and dried. Thisprocedure was followed by an identical dipping, rinsing and dryingprocedure in the clay suspension. After this initial bilayer (BL) wasdeposited, the same procedure was followed with only one-minute diptimes for subsequent layers. This procedure was repeated until thedesired number of bilayers (BL number) was achieved. For the foams andfabrics, the drying step involved wringing the water out prior toair-drying. Films with other polymers were made using the sameprocedure.

Film, Foam and Fabric Characterization

Film thickness was measured using a PHE-101 Discrete WavelengthEllipsometer (Microphotonics, Allentown, Pa.) at a wavelength of 632.8nm and a 65° incidence angle. Mass increase as a function of individuallayers deposited was measured using a Research Quartz CrystalMicrobalance (RQCM) (Maxtek Inc., Cypress, Calif.). Film absorbance wasmonitored at wavelengths between 190 and 900 nm using a USB2000-UV-VISSpectrometer (Ocean Optics, Dunedin, Fla.). Thin film cross-sectionswere imaged using a JEOL 1200EX TEM (Parbody, Mass.). Surface images ofcoated foams and fabrics, as well as of the chars from foams and fabrics(after heat-treatment or direct exposure to flame), were acquired with aQuanta 600 FE-SEM (FEI Company, Hillsboro, Oreg.).

Thermal Analysis and Vertical Flame Test

The thermal stability of uncoated and coated foams and fabrics wasmeasured in a Q50 Thermogravimetric Analyzer (TA Instruments, NewCastle, Del.). Each sample was run under air from room temperature to500° C., at a heating rate of 10° C. per minute. Vertical flame testswere conducted on untreated and treated fabrics according to ASTMD6413-08. An Automatic Vertical Flammability Cabinet model VC-2 waspurchased from Govmark (Farmingdale, N.Y.).

Results and Discussion

Influence pH on BPEI-MMT

The pH of an aqueous solution containing 0.1 wt. % BPEI was altered fromits natural value of approximately 10.4 using 1M HCl to pH 7, 8, 9 and10. Films were deposited by alternately exposing a substrate to the BPEIsolutions and an aqueous suspension containing 0.2 wt. % MMT, which wasnot altered from its natural pH of approximately 10. FIGS. 9a ), b)showed the linear growth exhibited by the combination of MMT clay withBPEI and the influence of BPEI pH on thickness. For films made with clayand polyethylenimine, thickness as a function of bilayers deposited wasshown in FIG. 9a ), and mass as function of individual layers depositedwas shown in FIG. 9b ). Thickness measurements were made with anellipsometer suing Si wafer substrates. Mass measurements were obtainedwith a quartz crystal microbalance. Filled data points in FIG. 9b )denoted polymer layers, and unfilled data points denoted clay layers.Such data show that the film growth was reduced by decreasing the pH ofthe BPEI solution. This was due to the high charge density BPEI has atlowered pH values, which caused the polymer backbone to repel itselfinto a flattened state. The opposite was true at high pH, where a lowercharge density and more coiled polymer resulted in thicker deposition. Adeposited mass per layer was measured with a quartz crystalmicrobalance, as shown in FIG. 9b ). While the QCM data confirm thelinear growth observed with the ellipsometer, it additionally revealedthe weight fraction of clay and density of each nanocomposite thin film.As expected, the thinner pH 8 film had greater clay concentration (81.6wt. %) and density (2.63 g/cm3) than its thicker pH 10 counterpart (79.2wt. % clay and ρ=2.1 g/cm3). This level of clay in a polymer compositewas unprecedented, especially in light of the fact that these films werecompletely transparent. (i.e., the clay was completely oriented andexfoliated).

Visible light absorbance as a function of bilayers deposited (FIG. 10a)) also revealed linear growth, but it also demonstrated the opticalclarity of these films. Percent transmission as a function of BPEI pH in40-bilayer films (FIG. 10b )) revealed the optical clarity control BPEIpH has on these films. In conventional clay-filled polymer composites,clay concentrations of 20 wt. % yield transmission levels of 79% atbest, but with LbL assembly 95% transmission was achievable with 81.6wt. % clay. For 40-bilayer films made with clay and PEI, absorbance at565 nm as a function of bilayers deposited was shown in FIG. 10a ), andpercent transmission as a function of polyethylenimine pH was shown inFIG. 10b ). Data was obtained using a UV Vis spectrometer.

TEM images of two 40-bilayer film cross-sections were taken. Individualclay platelets were seen as dark lines in the pH 10 film, which revealeda nano brick wall structure shown schematically. The images emphasizedthe high level of clay exfoliation and orientation, with all plateletslying parallel to the polystyrene substrate. Furthermore, these imagesverified the thickness measurements shown in FIG. 9a ) as the pH 8 filmwas approximately one half the thickness of the pH 10 film. Thicknessesof these films seemed greater than the ellipsometric measurements (FIG.9a )), because these cross-sections were often cut at an angle ratherthan perpendicularly through the films surface. The waviness of thesefilms was likely an artifact of the TEM sample preparation, whichfacilitated some stress relief in the films.

Example 3

Open-cell PU foams from NIST were coated with 10 and 40 bilayers of BPEI(at pH 8 and 10) and MMT. The onset of degradation and percent char at500° C., determined by TGA for different foam coatings, was shown inFIGS. 11a ), b). At 400° C., the highest char percentage was achievedwith the pH 8 BPEI coating (11.73 wt. % char from foam coated with 40bilayers). The coating made with BPEI pH 8 had higher clay concentrationand was more dense, which may protect the foam from further degradation.Based on the success of the TGA measurements (FIGS. 11a ), b)), largerfoam pieces with different coatings, as well as a bare one, were placedin an oven and heated up from room temperature to 400° C. and then heldfor 30 minutes to observe the degradation of foams. The uncoated foammelted and stuck to the ceramic crucible and could not be detached withtweezers. The char from the foams coated with BPEI (pH 8 or pH 10), andMMT was easily removed from the crucibles. Foam coated with pH 8 BPEIleft a thicker residue, which retained the open-cell structure of thefoam. Compared to the foam coated with pH 8 BPEI, foam coated with pH 10BPEI left a very thin, macroscopically porous layer.

Images of Foam Before and after Treatment

In order to get more information about the coating on the foams, SEM wasused to image the microstructure of the uncoated and coated foams. Threedifferent foams—uncoated, coated with BPEI (pH 8)/MMT, and coated withBPEI (pH 10)/MMT were imaged (FIG. 12). FIG. 12 showed SEM images ofbare and coated PU foams at different magnifications. The left columnwas a control foam. The center column was a foam coated with BPEI (pH8)/MMT. The right column was a foam coated with BPEI (pH 10)/MMT. Theopen cell structure for the three foam samples was seen very clearly;however, in the coated foams some particles were also seen. Undergreater magnification, the surface of the control foam was very smooth,unlike the coated foams, whose surfaces were covered by MMT, eitheraggregated or exfoliated into platelets. As expected, more MMT wasobserved on the foams coated with lower pH of the BPEI cationic dippingsolutions.

FIG. 13 showed SEM images of the PU foams after being heated in the ovenat 400° C. Control foams were shown in the left column. Foam coated withBPEI (pH 8)/MMT was shown in the center column. Foam coated with BPEI(pH 10)/MMT was shown in the right column. The control foam, with noprotective coating, melted during heating, causing the cellularstructure to disappear. In the case of the foam coated with BPEI (pH8)/MMT, it kept the open-cell foam structure. This foam allowed deeperstructure (below the surface) to be observed, which meant that thedegraded foam retained its volume and thickness. This was not so withthe foam coated with BPEI (pH 10)/MMT, which after degradation flattenedout and became a very thin layer, although it still held the open cellstructure. Under greater magnification, the roughness of the surfaces ofcoated foams was evident, which was attributed to the clay structuresleftover after the heat treatment.

Thermal Stability of Cotton Fabric

Thermogravimetric Analysis and Vertical Flame Test

Since the foam coated with BPEI (pH 8)/MMT showed the best stability andheat resistance, the same coating formulation was used for cottonfabrics. Fabrics were coated with 10, 20, and 30 bilayers of BPEI (pH8)/MMT and analyzed with TGA. At 500° C., under an air atmosphere, theuncoated fabric left less than 0.4 wt. %. The char weight percentagesfor the coated fabrics were much higher, and very close to each other(around 7 to 7.3 wt. %) for the three different numbers of bilayers(FIG. 14).

An equivalent set of coated fabrics was prepared for the vertical flametest (ASTM D6413). Still shots from video-recorded flame tests at eightseconds were taken after ignition. A more vigorous flame was observed onthe control fabric compared to the coated fabrics. Additionally, theflame on the control fabric grew faster. After burning, no controlfabric was left on the sample holder, but the three coated fabrics leftchar to different degrees.

SEM images showed the microstructures of the uncoated and coatedfabrics. The fiber surface in the control fabric appeared very clean andsmooth compared to the coated fabrics. Increased amounts of aggregatedMMT particles were seen on the fibers as the BL number of the coatingincreased. Each fiber of the fabric was at least partially, if notcompletely, covered by the clay coating.

After the vertical flame tests, the chars were also imaged by SEM.Because the control fabric was burned completely, its ashes were takenfrom the edge of the sample holder for imaging. Holes in the threadstrands of the control fabric, caused by burning, were seen very clearlyin the SEM images, as well as some fibrous residues that are no longerthe fabric fibers. In the case of the char from the coated fabrics, theweave structure was still maintained at the macro scale. At greatermagnification, a solid shield layer on the fibers was seen clearly inthe image of the 10 BL coating. It was possible that after burning athigh temperature, the MMT clay platelets sintered together after coolingdown, which accounted for not seeing aggregated MMT or the edges of theplatelets, but large continuous pieces of coating instead.

Conclusions

Layer-by-layer self-assembled films were successfully deposited on PUfoams and cotton fabrics. As shown by the TGA data and vertical flametest results, the coated foams and fabrics generated significant char.The stability of two LbL-coated substrates were clearly seen from theSEM images, before and after heat treatment.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations may be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for coating a substrate to provide aflame resistant substrate, comprising: (A) exposing the substrate to acationic solution to produce a cationic layer deposited on thesubstrate, wherein the cationic solution comprises cationic materials,and wherein the cationic materials comprise polyethylene oxide,polyallylamine, or any combinations thereof, and wherein the cationicmaterials do not comprise a clay; (B) exposing the cationic layer to ananionic solution to produce an anionic layer deposited on the cationiclayer to produce the flame resistant substrate having a bilayercomprising the anionic layer and the cationic layer, wherein the anionicsolution comprises a layerable material, and wherein the layerablematerial does not comprise a clay; wherein the substrate is a foam orfabric.
 2. The method of claim 1, wherein the layerable materialcomprises a colloidal silica, inorganic hydroxide, silicon basedpolymer, polyoligomeric silsesquioxane, carbon nanotube, graphene,anionic polymer, or any combinations thereof.
 3. The method of claim 1,further comprising repeating steps (A) and (B) to produce a plurality ofbilayers.
 4. The method of claim 3, wherein at least one of the cationiclayers comprises a cationic material that is different than the cationicmaterial present in the cationic layer of another bilayer and at leastone of the anionic layers comprises a layerable material that isdifferent than the layerable material present in the anionic layer ofanother bilayer.
 5. The method of claim 1, further comprising rinsingthe cationic layer, the anionic layer, or combination thereof.
 6. Themethod of claim 1, further comprising drying the cationic layer, theanionic layer, or combination thereof.
 7. The method of claim 1, furthercomprising adjusting the pH of the anionic solution, the cationicsolution, or both the anionic solution and the cationic solution.
 8. Themethod of claim 1, wherein the layerable material comprises an anionicpolymer, a colloidal particle, or any combinations thereof.
 9. A methodfor coating a substrate to provide a flame resistant substrate,comprising: (A) exposing the substrate to a cationic solution to producea cationic layer deposited on the substrate, wherein the cationicsolution comprises cationic materials, wherein the cationic materialscomprise polyethylene oxide, polyallylamine, or any combinationsthereof, and wherein the cationic materials do not comprise a clay; (B)exposing the cationic layer to an anionic solution to produce an anioniclayer deposited on the cationic layer to produce the flame resistantsubstrate having a bilayer comprising the anionic layer and the cationiclayer, wherein the anionic solution comprises a layerable material,wherein the layerable material does not comprise a clay; (C) repeatingsteps (A) and (B) to produce a plurality of bilayers; and wherein thesubstrate is a foam or fabric, and wherein at least one of the cationiclayers comprises cationic material that is different than the cationicmaterial present in the cationic layer of another bilayer, at least oneof the anionic layers comprises layerable material that is differentthan the layerable material present in the anionic layer of anotherbilayer, or any combinations thereof.
 10. The method of claim 9, whereinthe polymer comprises a cationic polymer, a polymer with hydrogenbonding, or any combination thereof.
 11. The method of claim 10, whereinthe polymer with hydrogen bonding comprises polyethylene oxide,polyallylamine, or any combination thereof.
 12. The method of claim 9,wherein the cationic polymer comprises branched polyethylenimine,cationic polyacrylamide, cationic poly diallydiemthylammonium chloride,or any combinations thereof.
 13. The method of claim 9, wherein thelayerable material comprises an anionic polymer, a colloidal particle,or any combinations thereof.
 14. The method of claim 13, wherein thecolloidal particle of the layerable material comprises a colloidalsilica, an inorganic hydroxide, a silicon based polymer, apolyoligomeric silsesquioxane, a carbon nanotube, a grapheme, or anycombination thereof.
 15. The method of claim 9, further comprisingrinsing the anionic layer, the cationic layer, or any combinationsthereof.
 16. The method of claim 9, further comprising drying theanionic layer, the cationic layer, or any combinations thereof.
 17. Themethod of claim 9, further comprising adjusting the pH of the cationicsolution, the anionic solution, or both the cationic solution and theanionic solution.